The invention relates to prostheses in general, and more particularly to improvements in hip prostheses of the type wherein a saddle-like head engages a pelvic bone and a shank is anchored in the cavity of the femur.
Prostheses of the above outlined type are popular and in widespread use, especially if the pelvic bone is damaged so that the acetabulum is useless and the head of the prosthesis must engage another portion of the hipbone. This can happen as a result of an accident or due to illness. As a rule, a substitute socket is formed in the lowermost portion of the remaining pelvis and the saddle-shaped head of the properly implanted prosthesis is maintained in sliding engagement with such substitute socket. The two horns of the saddle-shaped head of the prosthesis flank the substitute socket to prevent the head from leaving the socket. It is also possible to have the head of the prosthesis extend into a hole of the pelvis. Reference may be had to commonly owned U.S. Pat. No. 4,645,507 granted Feb. 24, 1987.
It has been found that heretofore known prostheses with saddle-like heads fail to satisfy all of the requirements which must be fulfilled by an artificial hip joint. First of all, when the wearer of the prosthesis is walking, relative movement between the head of the prosthesis and the socket of the pelvis entails a pronounced mechanical stressing of the remainder of the pelvis. When the pelvis and the femur perform large movements relative to each other, the horns of the saddle-like head of the implanted prosthesis strike the adjacent portions of the pelvis. Secondly, frictional engagement between the head of the implanted prosthesis and the adjacent portion of the pelvis brings about extensive wear upon the pelvis; in fact, the head is likely to penetrate into the pelvis and to shorten the respective lower extremity of the patient.
It was further discovered that, when the pelvis has undergone extensive damage (either as a result of an accident or as a result of illness), the muscles in the region of an implanted conventional prosthesis with a saddle-like head can exert only relatively small forces in a sense to straighten out the extremity into which the prosthesis is implanted because they are incapable of finding an appropriate lever arm for the application of conversion or transmission forces which are being generated thereby. Therefore, a patient wearing such a prosthesis is likely to limp because she or he must continuously strive to maintain the center of gravity of the body above the vertical axis which is common to the saddle-like head and the shank of the implanted artificial hip joint. Consequently, many patients prefer to forego or to prematurely interrupt therapies involving pronounced physical stresses which are necessary to save or prolong the life of a patient following a serious illness (such as the removal or partial removal of a malignant tumor) because the prospects of walking again, and especially of walking without a pronounced limp, are slim or nil.